This invention relates to radiation curable coatings for thermoplastic substrates. More particularly, the invention relates to UV-curable acrylic coating compositions and processes for coating thermoplastics, such as polycarbonates, with such coatings.
Thermoplastic sheets and film, such as those made from polycarbonates, polyestercarbonates, polystyrene, polyester and the like, find a wide variety of applications in the construction of interior and exterior signs, building materials, automotive and aircraft parts, and appliance parts, to name but a few. It is frequently desirable that the thermoplastic substrate have a glossy surface finish substantially devoid of ripples, scratches and other imperfections. On a commercial scale, obtaining such a surface finish on an untreated, extruded sheet or film is difficult. Moreover, the surfaces of untreated thermoplastic sheets and film are typically somewhat soft and easily scratched or marred and may exhibit a low chemical resistance.
To overcome these problems, thermoplastic substrates are often coated with a transparent protective coating. These coatings are designed to impart abrasion and chemical resistance to the substrates and may contain an ultraviolet light-absorbing compound to stabilize the substrate against UV-catalyzed degradation. A wide variety of such coating compositions have been proposed. Particularly useful coatings are based upon UV-curable acrylic polymer systems. For example, U.S. Pat. No. 198,465 to James E. Moore, et al., describes a polycarbonate article having a mar, abrasion and chemical resistant UV-cured coating comprised of the photoreaction products of certain polyfunctional acrylate monomers and resorcinol monobenzoate as a UV absorber. U.S. Pat. No. 4,188,451 to James S. Humphrey, Jr., describes a procedure for coating polycarbonate substrates which first uses a primer layer of a UV-cured polyfunctional acrylic ester monomer and an organic silicon compound and a top coating of a thermoset silica-filled organopolysixolane. U.S. Pat. No. 4,190,681 to Walter L. Hall, et al., describes a procedure for coating polycarbonate substrates with a primer coating comprised of the photoreaction products of certain polyfunctional acrylic monomers, and thereafter vapor depositing a thin glass top layer to the acrylic-coated substrate.
Thermoplastics are finding increasing applications in fields where thin-gage metals and other materials were previously used. For example, polycarbonate sheets are being used for the fabrication of appliance parts, automotive instrument panels, signs and the like. These applications often require the application of graphic arts or printing on the surface of the thermoplastic substrate. Commonly used inks and graphics materials (so-called "topcoats") are themselves UV-curable polymeric systems. A troublesome feature of acrylic-coated thermoplastics is that their surface is highly crosslinked and relatively inert; therefore, topcoats often do not adhere strongly to such surfaces.
A need exists for a coating composition for thermoplastic substrates, particularly polycarbonates, which imparts good abrasion and chemical resistance, and which also provides a surface for strong bonding of UV-curable topcoats.